The present invention relates to an electrophotographic printing plate that has high sensitivity over the entire range of visible light and which is adapted for use in an electrophotographic apparatus, notably in a platemaking system employing projection platemaking system for exposure to visible light, a laser platemaking system employing a gas laser as a light source, or a platemaking system using a light-emitting diode as a light source.
In the art of platemaking processes, active R&D efforts are being made on presensitized (PS) plates since they are lighter in weight, easier to carry and require simpler procedures for platemaking than conventional letterpress and intaglio plates. The technological aspects of the recent investigation of PS plates may be summarized as a capability for easy development, stability under illumination (light stability) and a higher sensitivity. The last-mentioned feature (higher sensitivity) is an objective which researchers are particularly interested to attain because a plate can be directly made from, a highly photoconductive material using an electric signal from a laser. However, most of the photoconductive materials currently used in PS plates of the film contact exposure type employ photochemical reactions and it is said that they must have a maximum sensitivity (i.e., the minimum amount of exposure necessary to form image) of at least 0.1 to 0.5 mJ/cm.sup.2. In comparison, no more than a hundredth of this amount of exposure is needed in electrophotographic platemaking processes which record image by making use of photoconductivity, and platemaking systems that employ the electrophotographic process have been introduced in the market.
Image-forming materials employed in such electrophotographic platemaking processes are classified as electrophotographic photoreceptors which are composed of substrates (e.g., metal plates, metal foils and paper) that are rendered hydrophilic by a suitable method such as anodization and which are provided with photoconductive layers having photoconductive materials dispersed in alkali-soluble binder resins. In order to make printing plates from such electrophotographic photoreceptors, they are first subjected to standard procedures of electrophotography, wherein a latent electrostatic image formed is rendered visible by application of an electroscopic toner and the resulting toner image is fixed by either heating or with a solvent vapor. In the next step, the non-image areas of the photoreceptor are washed away with an aqueous alkaline solution in case the toner is insoluble in aqueous alkaline solutions. Commercial organic photoconductors for platemaking currently available include organic dyes (e.g., Elfasol of Kalle A.G.) and organic pigments (e.g., EAC-2 of Polychrome Corporation).
The image recording process and the light source to be used differ depending upon the wavelength range of sensitivity of image-forming materials and their sensitivity. The image-forming materials developed so far have sensitivity in the short range of wavelengths and are unable to utilize exposure light with a very high efficacy. Therefore, in order to form image, they have to be exposed to visible light for a long time or, alternatively, they are exposed by scanning with an argon laser having an oscillation frequency at 488 nm. However, a long exposure time is not only inefficient but also suffering from damage to the original due to a heat or light energy irradiated by the light source. The argon laser requires large power consumption and the laser oscillating tube is relatively short-lived. Therefore, it is difficult to manufacture low-cost platemaking equipment that employs an argon laser as an exposing light source. Intensive studies are, therefore, being conducted in order to extend the wavelength range of sensitivity of photoreceptors to the longer side so that the exposure time can be shortened in platemaking processes and that a He-Ne laser whose oscillating tube has a prolonged life and which requires less power consumption can be used in exposure by laser scanning.
In modern platemaking systems using a He-Ne laser as a light source, photoreceptors are required to have a satisfactory sensitivity (e.g., no more than 5 .mu.J/cm.sup.2 in terms of halftime exposure) at a wavelength of 633 nm. Depending on the type of light source used and its light intensity, sensitivity in projection platemaking system requires the use of photoreceptors having practical halftime exposures of no more than 10 lux.sec. In platemaking from PS plates, it is common practice to expose many images separately on the plate surface or to expose a predetermined number of plates before they are transferred to a development station, so a certain mechanism is required that enables the operator to readily distinguish the exposed areas from the unexposed areas on the image wise exposed PS plate. In electrophotographic platemaking process that involves the formation of a toner image and the use of an alkali solution to wash away the non-image areas where no toner particles have been deposited, it is required that the toner image on the photoreceptor to be treated with an alkali solution should be clearly visible to the operator.
The image-forming materials that have been reported in the literature for use in electrophotographic platemaking processes are chiefly those which employ copper phthalocyanine compounds as photoconductive materials. However, because of the high optical density produced on the photoreceptor as a result of light absorption by phthalocyanine compounds, the reported image-forming materials have low contrast with the toner image, yielding a very low image visibility. As a further problem, the residual potential in the exposed areas is high enough to cause frequent fogging of the toner image.
For the purpose of producing image of improved quality in electrophotography, liquid developers are preferred over powder developers that are less efficient in achieving high resolution. But the stability of electrophotographic systems is also governed by the stability of developers as exemplified by dispersion stability and charge stability. Systems preferred in this respect are those which employ developers that retain stable positive charges and in order to use such developers, a negatively chargeable photoreceptor is necessary. The phthalocyanine compounds known in the art can be used to make positively chargeable photoreceptors but no negatively chargeable photoreceptors having high performance have yet been produced using such phthalocyanine compounds.